European Patent No. 108590 discloses a method and apparatus for installing optical fibre members within tubular ducts using fluid drag of a gaseous medium to propel the fibre member along the duct. In the illustrated examples in that patent, a fibre bundle is introduced to the duct via a feed head and wheels, in order to urge the bundle into the duct against a resistive force acting on the advancing end of the bundle.
A number of other methods of introducing fibres into ducts, for subsequent advancement along the ducts with the aid of viscous drag forces, have since been described.
One particular such method comprises introducing a fibre member to a duct at a point near a high local flow of compressed air, and bleeding some of the air through a vent some distance, typically around 1 meter, downstream. This method is fully described in our European Patent Application 0287225A1.
In all practical applications of the fibre blowing process, the nature of the fibre bundle to be installed has been an important factor. The present applicants and others working on the process have investigated a variety of different bundles, in an attempt to develop an optimum range of products. One product which has particularly good characteristics is described in our European Patent Application No. 0157610. One of the examples in that application is a seven-fibre bundle with a tight skin around the fibres, and an outer sheath of foamed material. The characteristics of this bundle are that it is lightweight, given its volume, and its large surface area provides a good surface for the action of fluid drag forces. There is the further advantage that the tight skin, or inner sheath, locks the fibres together, so that the rigidity is increased, and the fibres are unlikely to buckle. Recently, there has been a trend towards using more lightweight bundles with only a small number of fibres or even a single fibre. Generally for practical applications it has been thought important to create a package structure of some kind, usually comprising a foamed layer, to provide a surface area to weight ratio which results in the package being blowable over tortuous routes, which may extend over hundreds of meters. Typically, fibre members have been in the range 1 to 3.5 gm.sup.-1. Thus, a relatively bulky but light structure was seen as important for good blowing properties. Additionally, protection for the fibres was seen as essential, because any process carried out on fibres is seen as running some risk of damaging the fibres, for example by transmitting excessive tensile or compressive force to the fibres. Thus, robust packaging was used.
As is well known, the provision of a foamed layer in conjunction with optical fibres presents a number of practical difficulties, because shrinkage of the foam can introduce undesirable compressive forces which can adversely affect the transmission properties of the fibres. The above-described locking-effect of seven tightly encased fibres is one way of mitigating this problem. A loose layer of foam is another. Lengthy development has, however, been required to produce a robust product suitable for a wide range of working environments.